According to Beer’s Law, absorbance is directly proportional to the path length of the UV light through the water sample. Absorbance is given by the following equation:
UVA = ε•b•c
where ε is the molar absorbtivity of the particular type of matter in the water sample, b is the path length of the water sample, and c is the concentration of matter in the water sample.
From the Beer’s Law equation above, it is clear that if the path length is altered by a certain factor the UVA measurement will be altered by the same factor. Therefore, a UVA reading measured at a 25 cm path length can easily be converted to the UVA at a 1cm path length simply by dividing the 25 cm UVA measurement by a factor of 25. Real Tech takes care of this for you however. All Real Tech instruments display the UVA in terms of a 1 cm path length except for the Real UV254 M4250 Analyzer which displays the UVA in terms of a 1 m path length. Both 1 cm and 1 m path lengths are industry standard for absorbance measurements.